A method of measuring vibratory displacements in terms of a light wavelength
J. L. Goldberg · Journal of Physics E Scientific Instruments · 1968
A new method of determining the peak-to-peak displacement of a sinusoidal vibration in terms of the wavelength of light makes use of an interferometer of the Fizeau multiple-beam type, in which the silvered adjacent surfaces of the optical flats act as the two electrodes of a capacitor in addition to performing their function in the interferometer. The two electrodes are surrounded by a third or guard electrode so that the capacitance change between the silvered films which occurs when one of the optical flats is moved can be measured by comparison with a standard three-terminal capacitor in a transformer bridge. A sinusoidal motion is imparted to one surface of the interferometer and the capacitance change is recorded on a direct-writing oscillograph. This capacitance change is then calibrated by slowly moving the surface of the interferometer in a rectilinear manner and recording by a photoelectric detector the passage of interference fringes past a slit aperture in the optical field. The chart is thus calibrated in terms of interference fringes and the peak-to-peak displacement of the motion readily determined.